1. Field of Invention
This invention relates to a frictional damper. More particularly, this invention relates to a uni-directional frictional damper for self-activated spring-loaded pressure relief valves.
2. Description of the Prior Art
In attempting to avoid chatter and/or flutter and to insure valve stability during valve actuation, valve manufacturers have traditionally relied on the following three methods of damping:
viscous external damping involving the attachment of different hydraulic type viscous dampers to the moving parts;
viscous internal damping greasing ports having a relative movement during actuation;
viscous internal damping through special flow path design including damping orifices; and
frictional dampers.
Viscous dampers utilizing grease are relatively effective for small orifice valves at low set pressure. However, for the majority of valve applications, viscous damping has proven to be uneconomical due, in part, to the reliance of such dampers on a large damping area. In addition, viscous damping using grease is heavily dependent upon temperature and cycle count. Therefore, for many valve requirements, such damping means is unavailable and/or untrustworthy.
The damping forces created by viscous hydraulic dampers are proportional to the velocity of the moving parts. Accordingly, viscous hydraulic dampers are ineffective in preventing initiation of oscillation when the velocity of the moving parts is low.
Viscous damping through special flow path design, using the damping orifice approach, has also not led to any significant improvement over the aforementioned damping techniques.
Frictional damping (coulomb) presents another alternative which, although avoiding many of the problems associated with the control of damping fluids, presents many of its own problems such as worn components, jamming and close machine tolerance requirements.
U.S. Pat. No. 4,481,974 to Schmitt et al reveals a frictional damping means for damping valve vibration. In so doing, Schmitt et al provide a number of friction blocks positioned between wedging devices which are subjected to the compression force of a spring clamp. The Schmitt et al device represents a rather complicated frictional arrangement which relies heavily on specific angles being formed in the contacting components as well as careful choices in the types of springs being used. Thus, the Schmitt et al device is susceptible to the problems associated with wear and close tolerances, as well as high manufacturing and assembly costs. In addition, Schmitt et al, which rely on sliding contact between various friction surfaces, presents the additional problem of having the friction blocks becoming inoperative.